This protocol was designed to estimate postnatal longitudinal growth in infants with birth weights from 501 through 1500 grams, admitted to the NICU before 24 hours of age, and free of congenital anomalies. The need for growth curves for this unique population has become apparent as greater numbers of low birthweight infants survive. Combining data from this study with the NICHD Neonatal Generic Data Base provided an excellent opportunity to assess the nutritional process and relationships between velocity growth and nutritional support, velocity growth and severity of illness, and velocity growth among those infants who develop the problems for which this population is at high risk, such as chronic lung disease.